Policy for minimum versions on dependencies

There's a [discussion over in the Dask issue tracker](https://github.com/dask/dask/issues/7378) around adopting a new policy for the minimum version of dependencies Dask should support. Currently the proposal is "we support versions which were released in the last 18 months".

This would mean, for example if we adopted this policy today, our minimum supported version of NumPy and pandas would be:

- `pandas >= 0.25.2` (our current minimum is `0.25.0`)
- `numpy >= 1.17.2` (our current minimum is `1.15.1`)

I'm bringing this up in the community issue tracker in case there are people here who might have thoughts on such a policy but don't watch the `dask/dask` issue tracker. If you have thoughts on this topic, in particular if adopting such a policy would negatively impact you, I encourage you to engage over in https://github.com/dask/dask/issues/7378.

Read the linked Dask discussion at dask/dask#7378 first, focusing on the proposed 18-month support window and its impact on the stated NumPy and pandas minimum versions. Done would require a resolved policy decision and corresponding documentation or project changes, but this community issue directs discussion elsewhere rather than naming an implementation entry point.

Written by the indexing model from the issue text.